It is proposed to further elucidate the nature of the cerebellar lesion found in the "weaver" mouse. It has been shown that in these cerebella the granule cells and the parallel fibers are virtually absent. Their usual postsynaptic mates, however, the Purkinje cell dendritic spines, are present and practically indistinguishable, on a morphological basis at least, from those found in the normal mouse. The proposed work is designed to explain the histogenesis of these "unattached" dendritic spines to determine whether they develop without the influence of the parallel fibers or they are the remains of a degenerated synapse. In addition, the cholinesterase activity of this area in the weaver is being explored histochemically and compared to that of the normal mouse.